This invention concerns cathodes, particularly drained cathodes for alumina reduction cells, which present a refractory surface for contacting molten aluminum on the cell floor. This invention also concerns materials for constructing such cathodes.
Presently, aluminum metal is conventionally produced by the electrolytic reduction of alumina dissolved in a molten cryolite bath according to the Hall-Heroult process.
This process for reducing alumina is carried out in a thermally insulated cell or "pot" which contains the alumina-cryolite solution. The cell floor, typically made of a carbonaceous material, provides some of the thermal insulation and serves as part of the cathode. The cell floor may be made up of a number of carbonaceous blocks bonded together with a carbonaceous cement, or it may be formed using a rammed mixture of finely ground carbonaceous material and pitch. The anode, which usually comprises one or more carbonaceous blocks, is suspended above the cell floor.
Resting on the cell floor, there is a layer or "pad" of molten aluminum which the bath sees as the true cathode. The anode, which projects down into the bath, is normally spaced from the pad at a distance of 1.50 to 3.00 inches (3.81 to 7.62 centimeters). The alumina cryolite bath is maintained on top of the pad at a depth of about 10.00 to 12.00 inches (25.40 to 30.48 centimeters).
As the bath is traversed by electric current, alumina is reduced to aluminum at the cathode, and carbon is oxidized to its dioxide at the anode. The aluminum thus produced is deposited on the pad and is tapped off periodically after it has accumulated.
For the electrolytic process to proceed efficiently, the alumina reduction should occur on to a cathode surface of aluminum and not the bare carbonaceous surface of the cell floor. Therefore, it is considered important for the pad to cover the cell floor completely.
As molten aluminum does not readily wet or spread thin on carbonaceous materials, the pad can best be visualized as a massive globule on the cell floor. In larger cells, the heavy currents of electrolysis give rise to powerful magnetic fields, sometimes causing the pad to be violently stirred and to be piled up in selected areas within the cell. Therefore, the pad must be thick enough so that its movements do not expose bare surface of the cell floor. At the same time, the anode must be sufficiently spaced from the pad to avoid short circuiting and to minimize reoxidation of aluminum.
Still, the movements of the pad have adverse effects which cannot be readily controlled. For a given cell operating with a particular current of electrolysis, there is an ideal working distance between the cathode and the anode for which the process will be most energy efficient. However, the required spacing of the anode due to the turbulence of the pad prevents this ideal working distance from being utilized. Further, since the pad is in a state of movement, a variable, uneven working distance is presented. This variable working distance can cause uneven wear or consumption of the anode. Pad turbulence can also cause an increase in back reaction or reoxidation at the anode of cathodic products, which lowers cell efficiency. In addition, pad turbulence tends to accelerate bottom liner distortion and degradation through thermal effects and through penetration by the cryolite and its constituents.
It has been suggested in the literature and prior patents that certain special materials, such as refractory hard metals (RHM), and most notably titanium diboride (TiB.sub.2), can be used advantageously in forming the cell floor.
Ideally, in contrast to conventional carbon products, these materials are chemically compatible with the electrolytic bath at the high temperatures of cell operation. They are also compatible chemically with molten aluminum.
Also, with these special materials, the electrical resistance across the interface between the molten aluminum and the cell floor is much lower than where the cell floor is formed by bare carbon. Thus, it should be possible to operate the cell with reduced electrical power requirements.
Furthermore, the special cell floor materials are wetted by molten aluminum. Accordingly, the usual thick metal pad should no longer be required, and molten aluminum may be maintained on the cell floor as a relatively thin film. This can be conveniently carried out using a "drained cathode" configuration where molten aluminum is continuously drained off of the cathode as the aluminum is electrolytically reduced.
By using a drained cathode design, the substitution of a relatively thin film of molten aluminum for the conventional metal pad eliminates a source of electrical resistance in the cell. In addition, the anode-cathode working distance across the bath can be shortened considerably by going to a drained cathode design, which would reduce the electrical resistance of the cell still further, and would also permit the most efficient anode-cathode working distance to be utilized.
In general, the prior art has recognized the potential of these special RHM cell floor materials to improve cell operating efficiency and reduce electrical power requirements, particularly in conjunction with the employment of drained cathodes. However, there are still other benefits to be derived from the use of these RHM materials.
One such benefit relates to the tendency in conventional cells to experience sidewall erosion, which can sometimes go so far as to result in a serious leak requiring the operation of the cell to be shut down. In a typical cell where this happens, the sidewalls comprise an interior liner built up from carbon products much in the same way as the cell floor. The problem arises as carbon in this liner reacts with molten aluminum in the metal pad to form an aluminum carbide film or layer on the sidewall surfaces in contact with the pad. Once formed, this aluminum carbide film or layer acts as a barrier to further reaction. Aluminum carbide, however, is soluble in molten cryolite. Thus, as the carbided sidewall surfaces are brought into contact with the bath, the aluminum carbide film or layer dissolves and washes away, exposing bare sidewall carbon for reaction with aluminum. In the typical conventional cell, the sidewall carbon is constantly subject to alternate contact with cryolite and molten aluminum, due to pad turbulence, for example, or to the rise and fall of the pad height as produced aluminum accumulates and is subsequently tapped off. In the course of such alternate cryolite and molten aluminum contact, the sidewall liner carbon is gradually eroded away.
Obviously, however, if the pad is eliminated by going to a drained cathode design, sidewall erosion should no longer be a problem.
Another benefit to be derived from the use of the special RHM cell floor materials is an expected increase in the feasibility of using graphite as a material of construction for the cell floor.
As contrasted with conventional cell floor materials, such as anthracite or mixtures of anthracite and graphite, graphite has a considerably higher electrical conductivity. Also, graphite is a superior material from the standpoint of resistance to growth and expansion, and to cracking, due to thermal effects and chemical attack in the reduction cell environment.
There are, however, certain problems to be associated with the use of graphite. For one thing, it is more expensive than the conventional materials. In addition, it is subject to being eroded through mechanical abrasion by the cell contents and particularly by the abrasive action of a turbulent metal pad.
But, if a graphite cell floor can be suitably covered by the special RHM materials, all of these problems, save the one of expense, can be minimized, if not eliminated. Since molten aluminum would primarily contact the highly wettable RHM materials, the relative non-wettability of the graphite would not be important. Since the graphite would be substantially covered by the abrasion-resistant RHM materials, it would not itself be significantly exposed to the mechanical forces of abrasion. Further, if a drained cathode design is employed, then the conventional metal pad will be eliminated as the chief source of harmful abrasive action.
While some of the potential benefits of using the RHM materials in cell floor constructions have been recognized for some time, in practice they have been elusive and most difficult to attain.
In the more serious prior art proposals, the cell floor is built up using conventional techniques with carbon products to form an electrically conductive bed. Embedded in this carbonaceous bed are a number of current collector bars, usually made of steel, but occasionally of RHM materials, which protrude from the cell for connection to the external electrical power source.
The configuration of the cell floor has been variously shown as a conventional horizontal surface or, preferably, as a drained cathode design. For a drained cathode, the cell floor may comprise one or more inclined surfaces each of which leads to a channel or a well to collect the drained metal. Alternatively, the cell floor may comprise a horizontal surface with one or more holes, passages, inclined grooves or channels to effect the cathode drainage.
The prior art has suggested a number of ways to employ the special RHM cell floor materials to complete the construction of these various cell floor designs. Typically, the special materials are employed as shapes, such as bars or plates, or even as small particles, made either by hot pressing or by cold pressing and sintering RHM powders.
In some of the prior art proposals, RHM bars or other elements, such as bars or elements made of titanium diboride-aluminum nitride (TiB.sub.2 --AlN), are situated deep within the structure of the cell floor or within the sidewalls of the cell, and extend into the cell interior to make contact with the electrolytic bath. These bars or elements may terminate within the floor or wall structure and act simply as electrically conducting elements. Or, they may terminate outside of the cell and function as current collector bars. Or, they may terminate within the floor or wall structure and there connect with current collector bars, which in turn terminate outside of the cell.
One objection to this type of arrangement is that the RHM materials will usually be subjected to adverse temperature gradients and corrosion and will tend to fracture. While RHM materials have very high compressive strength, they are quite brittle and pressed shapes often become mechanically unstable if exposed to thermal shock. Additionally, in alumina reduction cells, elemental alkali, mostly in the form of sodium, tends to permeate the porous materials of the cell liner. At temperatures of about 880.degree. C., which exist within the cell liner, the sodium begins to form compounds, including eutectic compounds, with the bath constituents. Experiments have shown that the formation and crystallization of these compounds will tend to corrode and fracture an RHM element situated within the cell liner in the vicinity of the 880.degree. C. isotherm.
Another objection to this type of arrangement is that the cathode does not comprise the continuous, flat surface required for optimum electrolytic efficiency and even consumption of the anode.
In accordance with other prior art proposals, RHM particles are contained in a carbonaceous binder and applied as a layer to the cell floor. An objection to this type of arrangement is that the carbonaceous binder material is commonly subject to chemical attack, and the expected service life is correspondingly low. Another objection is that the presence of the binder decreases the degree of wettability by molten aluminum.
It is accordingly believed that a better and more practical arrangement should incorporate discrete RHM shapes, such as tiles, fastened or bonded to a built up carbonaceous bed to define a cell floor and cathode.
Each of these RHM shapes should be employed in a substantially uniform temperature region within the cell so as to avoid thermal shock. Hence, the shapes should be relatively thin in the sense that they will not protrude significantly either into or out from the cell wall or floor surfaces to which they are attached.
The RHM shapes should further be employed within the hotter regions of the operating cell and away from the 880.degree. C. isotherm.
Furthermore, it is preferred that the shapes be employed so as to substantially cover the cell floor and provide a cathode characterized by essentially continuous flat refractory surfaces for contacting molten aluminum.
While the construction of such a cell has been heretofore contemplated by the prior art, the major problem has been to find a way to satisfactorily fasten or bond the shapes to the carbonaceous cell floor.
One requirement is that at cell operating temperatures, e.g., about 930.degree. to 975.degree. C., the bonding means should provide a good electrically conducting connection between the RHM shapes and the carbonaceous substrate. This suggests the possibility of using carbonaceous pastes or cements, such as are conventionally used to build a cathode structure with carbon blocks.
Another requirement, however, is that the bond should be strong mechanically. More particularly, it should be compatible with installing the RHM shapes at room temperature followed by heating the cell to the high temperatures of operation.
However, experiments have shown that RHM materials, i.e., TiB.sub.2 and TiB.sub.2 --AlN, will not consistently stay bonded to a carbonaceous substrate in a reduction cell operating environment when such pastes or cements are used.
It was against this background that this invention was made.